AssistedInject

Assisted injection is a type of injection where some dependencies may be fulfilled by the host dependency graph but some others may be assisted at runtime during instantiation. This is useful for deferring some inputs to dynamic values supplied at runtime.

For example, an HttpClient may accept a user-preferenced timeout duration.

@AssistedInject
class HttpClient(
  @Assisted timeoutDuration: Duration,
  cache: Cache,
)

In this scenario, you would then also define a AssistedFactory-annotated type (usually a nested class) to create this. This factory's requirements are defined on the AssistedFactory kdoc.

@AssistedInject
class HttpClient(
  @Assisted timeoutDuration: Duration,
  cache: Cache,
) {
  @AssistedFactory
  fun interface Factory {
    fun create(timeoutDuration: Duration): HttpClient
  }
}

This factory can then be requested as a dependency from the graph and used to instantiate new HttpClient instances.

@DependencyGraph
interface AppGraph {
  val httpClientFactory: HttpClient.Factory
}

fun main() {
  val httpClientFactory = createGraph<AppGraph>().httpClientFactory
  val httpClient = httpClientFactory.create(userPrefs.requestTimeoutDuration)
}

You can (and usually would!) access this dependency in any other injection site too.

Note: Assisted injected types cannot be scoped and can only be instantiated by associated AssistedFactory types.

See the docs on Assisted and AssistedFactory for more details on their use.

Top-Level Function Injection

Assisted injection is supported in top-level function injection, no need to use @AssistedInject annotation. See Inject for more details.